1. Technical Field of the Invention
The present invention relates to techniques for testing optical links involving wavelength division multiplex (WDM) systems. More particularly, and not by way of any limitation, the present invention is directed to a wavelength adaptive or agile optical transponder for testing a bi-directional, single-fiber optical link using a WDM system.
2. Description of Related Art
As networks face increasing bandwidth demand and diminishing fiber availability in the existing fiber plant, network providers are migrating towards a new network technology called the optical network. Optical networks are high-capacity telecommunications networks comprised of optical and opto-electronic technologies and components, and provide wavelength-based services in addition to signal routing, grooming, and restoration at the wavelength level. These networks, based on the emergence of the so-called optical layer operating entirely in the optical domain in transport networks, can not only support extraordinary capacity (up to terabits per second (Tbps)), but also provide reduced costs for bandwidth-intensive applications such as the Internet, interactive video-on-demand and multimedia, and advanced digital services.
Of the several key enabling technologies necessary for the successful deployment of optical networks, wavelength division multiplexing (WDM) has emerged as a crucial component for facilitating the transmission of diverse payloads regardless of their bit-rate and format over the optical layer. WDM increases the capacity of embedded fiber by first assigning incoming optical signals to specific wavelengths within a designated frequency band (i.e., channels separated by a predetermined spacing) and then multiplexing the resulting signals out onto a single fiber. Because incoming signals are not terminated in the optical layer, the interface is bit-rate and format independent, allowing service/network providers to integrate the WDM technology with existing equipment in the network.
By combining multiple optical signals using WDM, they can be amplified as a group and transported over a single fiber to increase capacity in a cost-effective manner. Each signal carried can be at a different transmission rate (e.g., Optical Carrier (OC)-3, OC-12, OC-48, etc.) and in a different format (e.g., Synchronous Optical Network (SONET) and its compani22on Synchronous Digital Hierarchy (SDH), Asynchronous Transfer Mode (ATM), Internet Protocol (IP)-based data or multimedia, et cetera).
Current advances in WDM technologies allow a plurality of wavelengths to be multiplexed over a fiber using nanometer and sub-nanometer spacing (Dense WDM or DWDM). For example, up to 32 channels or carriers may be spaced 100 GHz apart (equal to 0.8 nm) in a multiplexed optical signal operating in a particular transmission band. In contrast, some of the standardized, “coarse” wavelength separations include 200 GHz spacing (1.6 nm) and 400 GHz spacing (3.2 nm).
In a typical implementation of an optical link, usually a single optic fiber is deployed between two end points, e.g., an optical network unit or ONU and a head end located at an end office. The optical link is operable to carry both upstream and downstream signals within separate transmission bands of the single optic fiber in order to avoid signal conflicts, crosstalk and the like. This practice is generally referred to as broadband WDM.
Optic fibers composed of silica have three useful transmission bands located at about 850, 1310 and 1550 nm, which may be referred to as the 850 band, the 1310 band and the 1550 band. The existence of these bands is partly a function of the characteristics of the fiber itself, including such factors as the amount of optical absorption and dispersion within the fiber at different wavelengths, and partly a function of practical limitations on the availability of suitable devices, such as lasers and LEDs, used for coupling light into the fiber at different wavelengths.
As deploying optical links with WDM transmission capabilities has become prevalent, various techniques for testing such links in an efficient manner are being developed. Whereas several techniques for testing the optical link are currently available, the existing solutions are beset with various deficiencies and shortcomings.
For example, several wavelength-specific optical transponders are typically required for monitoring the path/performance integrity of an optical link capable of transmitting multiple wavelengths on a single fiber. In addition, it is often necessary for a technician to know beforehand the wavelength of the incident optical signal used.
Moreover, the current techniques do not provide for automatic rate adaption with respect to the multiple transmission rates available on optical links today. Typically, a plurality of devices are used for testing the link at different transmission rates. Further, a priori knowledge regarding the directionality of transmission is assumed where bi-directional transmission of the optical signals is employed.
Based upon the foregoing, it should be apparent that there has arisen an acute need for a system and method for testing an optical link, particularly a link formed of a bi-directional single fiber system involving WDM transmission, that overcomes these and other deficiencies and drawbacks in an efficient manner.